1. Field of the Invention
This invention relates to the fabrication of MicroElectroMechanical Systems (MEMS) devices and more specifically to a hybrid fabrication technique in which MEMS devices are formed using thin-film surface micromachining on a thick release process.
2. Description of the Related Art
MEMS is a relatively new technology, which exploits the existing microelectronics infrastructure to create complex machines with micron feature sizes. These machines have both electrical and mechanical components and can have many functions including sensing, communication and actuation. MEMS are useful because they are physically small and can be very precise.
MEMS fabrication can be separated into two classes: bulk micromachining which is a subtractive fabrication that converts the substrate into the mechanical parts of the MEMS device and surface micromachining which is an additive technique that involves building the device on the surface of the substrate. Surface micromachining is further refined into classic thin film processes in which the release layer and devices are constrained to no more than a few microns and electroplating-based processes such as LIGA in which the devices can be tens to hundreds of microns thick. LIGA processing forms a high aspect ratio mold on the surface of the substrate, which is subsequently filled with metal using electroplating techniques, resulting in thick metal structures built right on the surface of the substrate.
Classic thin film surface micromachining, historically originated from the semiconductor industry, is an additive fabrication technique which involves the building of a device, layer by layer, on top of the surface of a supporting substrate (e.g., a single crystal silicon wafer). The nature of the deposition processes involved, e.g., low-pressure chemical vapor deposited (LPCVD) and thermal oxidation, restricts practical layer thicknesses to no more than a few microns. These small vertical dimensions, i.e. height, may be a drawback for some devices and an advantage for others. The materials used for fabrication of layers of surface micromachining devices include doped and undoped polysilicon, silicon nitride, for the electrical and mechanical structures, and silicon dioxide, as a release material; and aluminum and gold alloys for the metal connections to the outside world.
In case of surface micromachined structures built on a silicon substrate, silicon dioxide is classically used as a release material, and is grown thermally. For example, a silicon wafer is placed in a water vapor ambient at 1000 degrees Celsius for 1 hour, which converts 0.3 microns of the silicon surface into 0.6 of silicon dioxide. Thermal oxide thickness is limited to a few microns due to the diffusion of water vapor through silicon dioxide. Silicon dioxide can be deposited without modifying the surface of the substrate, but this process is impractical due to the excessively long deposition time and the film is highly stressed. The result is films limited to 4 microns or less. Silicon nitride may also be used as a release material, but since its deposition process is the same, its thickness is also limited to the same range.
Many combinations of possible structural material and sacrificial layer can be, and have been, tried; examples are combinations of silicon nitride and polysilicon, gold and titanium, nickel and titanium, polyimide and aluminum, tungsten and silicon dioxide, and aluminum and polymer. The majority of the surface micromachining work has, however, focused on the combination of polysilicon as the structural material and silicon dioxide or related glasses as the sacrificial material.
Thin film surface micromachining is particularly well suited to forming very thin structures. Using special fabrication techniques low stress micro-structures can be obtained. As such thin film processing is preferred to form (1) actuators that have low voltage compliance and (2) sensors that must be able to respond to very small signals. Thin film processes have also aggressively explored for optical MEMS applications such as switches for all-fiber telecommunication networks. For many applications, however, the primary drawback of conventional surface micromachining technology is the limitation imposed by the height of the release layer (4 microns maximum). Such close proximity of devices to the substrate often results in stiction, which is a serious problem in these applications, and the design space is quite limited.
A separate approach in surface micromachining is to build tall structures, sometimes with high-aspect ratios. One method of this approach is LIGA, which is the German acronym for X-ray lithography (X-ray LIthographie), electrodeposition (Galvanoformung), and molding (Abformtechnik). The process involves a conductive substrate, a thick layer of X-ray resist (microns to millimeters,), high-energy X-ray radiation exposure and development to arrive at a three-dimensional resist structure. Subsequently, electroplating fills the vacant areas of the resist mold with a metal and, after resist removal, a freestanding metal structure results. The metal shape may be a final product or serve as a mold for precision plastic injection molding. Injection-molded plastic parts may in turn be final product or lost molds. Tiny (meso-scale) magnetic motors and precision-engineered gears are typical applications for the LIGA process. LIGA is not generally applicable to form sensors, actuators and optical devices because of the poor surface flatness of the thick layer, often surface height variations are larger that the thickness of the thin device structures required.
A family of LIGA-like processes enables movable microstructures. The ability to make movable LIGA devices is enabled by an addition of a thin sacrificial layer, between wafer substrate and the thick layer of LIGA resist. This allows the molding process to fabricate thick structures either partially attached or freely suspended slightly above the substrate. The sacrificial layer may be polyimide, silicon dioxide, polysilicon, or some other metal. Thus, this LIGA-like technology enables thick freely moving structures (thick thanks to the laminated film and freely moving credited to the underlying thin release layer).
Bulk micromachining is a subtractive fabrication that converts the substrate into the mechanical parts of the MEMS device. As such, the fabrication of a broader class of tall structures is easier than with classic surface micromachining. This is because the substrates can be thick resulting in relatively thick unsupported devices. Packaging of bulk micromachined devices tends to be more difficult than packaging surface micromachined devices. Furthermore, the removal (etching away) of thick layers of the substrate bulk does not easily lend itself to the controlled fabrication of very thin low stress structures required for low voltage operation that might be obtained with surface micromachining techniques.
In view of the above problems, the present invention provides a technique for monolithic fabrication of low voltage, IC compatible MEMS devices with the capability for low stress thin surfaces in a manner that expands the design space to include low voltage actuation and large out-of-plane actuation. Additionally, the proposed technology solves one of the drawbacks of surface micromachining, namely stiction yield, by eliminating physical contact of the device with the substrate. Furthermore, moving the devices away from the substrate reduces parasitic capacitance.
This is accomplished with a hybrid process that combines a thin-film surface micromachining process such as by sputtering, evaporation or chemical vapor deposition with a thick-film surface micromachining and release process such as lamination. Such combination results in thin film micro-structures, with all the benefits of surface micromachining that are significantly distant from the substrate.
The thick-film process forms a base on which the thin film devices are fabricated. The base comprises a thick release layer patterned in an appropriate manner to define a post array formed by, for example, electroplating or some other compatible process. The thin-film surface micromachining process then forms a device or devices on top of the thick base that, when released, are supported by the posts high above the underlying substrate. The thin film surface micromachining process may or may not involve an additional release layer. However, if an additional release layer is employed, a low temperature polymer based process is preferred to maintain compatibility with the thick-film process and, if used, IC wafer.
In one particular embodiment, the hybrid process includes the following steps:
(a) Preparing the substrate with patterned or unpatterned conductive layers;
(b) Laminating and then patterning a dry film to form the thick release;
(c) Electroplating the device to form the posts in the patterned areas of the dry film;
(d) Forming a MEMS device on top of the thick release supported by the posts; and
(e) Releasing the dry film.
Furthermore, due to the low temperature nature of this process the MEMS devices can be monolithically fabricated on top of an IC wafer. The IC wafer may be preprocessed using thin-film technology to form actuation electrodes or other electrical components on its surface. This hybrid process is particularly well suited for applications such as optical switches, scanners, sensors and actuators that could benefit from an increased range of motion.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: